1) Log into your admin.
2) Go to "Settings"
3) Go to each of the email settings for the 5 different emails that the site sends
4) make sure the "FROM" setting for each of the emails comes from your domain. That means if you aren't using a custom domain for your site it should be similar to: "firstname.lastname@example.org" or if you are using a custom domain you can use any working email address using your domain like: "email@example.com"